Does the die-back of aquatic plants enhance algal blooms that contaminate drinking water?

Aquatic plants are natural weapons to control algal blooms by competing with them for nutrients and light. Algal blooms can be problematic, for instance, they can create toxins that are harmful in drinking water.

It has been overlooked water level changes could kill aquatic plants and create a new nutrient source that would feed algal blooms (see Figure). This could be a problem for managing drinking water supplies.

Schematic for how water level changes affect nutrient release from aquatic plants. The solid red arrows show the role of aquatic plants as a nutrient source, and the hollow blue arrows show the role of aquatic plants as a nutrient sink

We examined the fate of released nutrients (focusing on nitrogen) from decayed aquatic plants after a drying-rewetting cycle. This was done by labeling plants with safe chemical tracers (stable nitrogen isotopes) and monitoring the nitrogen release from decayed plants after rewetting.

We found that algae utilized the nitrogen from decayed plants much faster than their competitors: the living plants that regrew after rewetting. However, the regrowth of plants after rewetting did reduce the N release to the water column and sediment.

The decomposition of an exotic submerged plant caused more nutrient release, algal growth and a decline of oxygen in the water, compared to a native plant. Sediment desiccation process (from water level drawdown) also increased water quality problems from decayed plants after rewetting, compared to the system without desiccation.

Experimental set-up. Photo: Dr Lu.

These findings indicate that effective water level management is essential to protect aquatic plants, especially the native ones, and reduce the risk of algal blooms during water level changes.

Algal blooms might be reduced by shortening the duration, intensity, or frequency of water level drawdown, or by removing the dead plants to reduce nutrient release after water refilling.